Benzaldehyde is an important starting material in various chemical syntheses, including those relating to the synthesis of scents and flavors. In these applications the benzaldehyde is often required to have a high degree of purity, but unfortunately crude benzaldehyde, and especially benzaldehyde prepared by the oxidation of toluene with a gas containing molecular oxygen will contain certain impurities that are very difficult to remove. One very significant problem presented by these impurities is that it is particularly difficult to obtain a product from such crude benzaldehyde that will satisfy olfactory specifications. Furthermore, the presence of such impurities also causes a quite rapid discoloration of the benzaldehyde during storage. Such discoloration will occur even at very low concentrations of the impurities, such as a few p.p.m. by weight. It is of interest to note that benzyl hydroperoxide is not normally present in the crude benzaldehyde in any significant quantities.
One suggested solution which appears in Japanese Pat. Publication No. 24,467/74 is to purify the crude benzaldehyde by treating it with an aqueous solution of sodium hydroxide. However, this method of purification does not give satisfactory results, as shown, by the fact that benzaldehyde treated in this manner is still found to discolor quite rapidly.
One method which does give satisfactory results is that disclosed in U.S. Pat. application Ser. No. 952,609 filed Oct. 18, 1978. The process disclosed in that application employs an oxidizing agent and a distillation step to accomplish the purification.
Still another method is disclosed in a sister application to the present application filed on the same date in the United States Patent Office. In that application, the difficulty was in trying to purify an impure benzaldehyde in the presence of water. That problem was overcome by treating the impure benzaldehyde simultaneously with water and a metal less noble than hydrogen followed by a distillation step.